The signal propagation time in the metallization planes of integrated circuits (ICs) is of critical importance to the performance of ICs. Particularly in the case of the new generation of chips, this propagation time plays a critical role in the overall speed of the chip, since the length of the metallic conductors is becoming increasingly greater in these chips. To counter the resulting increase in the total resistance of the interconnects, the interconnects used in logic chips are at present made not of aluminum but of copper, since the latter has a lower electrical resistance.
The propagation time is then influenced not by the resistance of the conductor, but by the dielectric constant of the insulator (=dielectric), e.g. between the interconnects. The smaller the dielectric constant, the shorter the propagation time. The coming generation of chips will require, in particular, materials having a dielectric constant of 2.5 or less.
In addition, the materials which are to be used as insulators in chip technology have to meet further strict criteria. These are, for example, high temperature stability (>450° C.) and low moisture absorption (<0.5%) together with very good adhesion to the substrates used in chip technology, for example silicon, silicon oxide, silicon nitride, etc. The abovementioned criteria have hitherto been met by porous organic materials whose matrix is a high-temperature-stable polymer. The “pores” have to have a size ranging from a few nanometers to a few tens of nanometers, since the structure dimension in IC technology has already reached the 100 nm mark.
A critical factor in the production of such porous insulators is the availability of a simple, fast and inexpensive process, i.e. a process which meets manufacturing needs.
According to the prior art (U.S. Pat. No. 5,777,990), a proposed solution is to use copolymers which comprise thermally stable and thermally labile blocks. The decomposition of the labile blocks at elevated temperature (“foaming”, usually at above 200° C.) enables pores to be generated, so that the dielectric constant of the material is reduced. By means of the this foaming process, the dielectric constant can be reduced to below 2.5.
The process described in U.S. Pat. No. 5,776,990 for generating pores by thermal decomposition of the labile block has various disadvantages. The thermal decomposition usually forms gaseous fragments which are toxic and/or corrosive and thus pollute or endanger the apparatus and the environment. A further significant problem is that these fragments partially dissolve or swell the stable block and can thus lead to collapse of the structure. In addition, such a decomposition process requires a thermal treatment for a period of up to 10 hours, which does not allow effective and inexpensive production. Furthermore, the stable block is also partially attacked during the decomposition of the labile block at elevated temperature, e.g. by free radicals which are formed in the labile block, as a result of which the stable block looses its thermal and mechanical stability. In addition, the block and graft copolymers described in U.S. Pat. No. 5,776,990 have to be prepared in a new synthesis for each ratio of labile to stable block which is required. In many cases, toxic materials such as phosgene or cresol have to be employed.
WO/0104954 A1 discloses a process for applying a layer to a substrate, in which a polymer composition is applied to a substrate and the solvent used is simultaneously or subsequently removed by the action of heat. Porosity is not described.
EP 881 678 A2 discloses a process for producing integrated circuits, in which silicon dioxide xerogel is applied to a substrate and residual solvent is removed by the action of heat to generate porosity. The xerogel displays poor adhesion to the substrate. In JP 10092804 A (patent Abstracts of Japan), WO 00/43836 A1 and XU,Y. et al.: Dielectric property of a porous polymer material with ultralow dielectric constant, Appl. Phys. Lett., 1999, Vol. 75, No. 6, pp. 853-855, porosity is generated in a layer adhering to a substrate by removal of a solvent from a polymer composition by means of thermal treatment.
WO 00/64953 discloses a process in which the inorganic part is leached from a mixture of organic polymer compounds and inorganic compounds so as to produce an organic polymer containing pores. A process for producing a layer adhering to a substrate is not disclosed.
It is therefore an object of the present invention to provide a process which does not have the abovementioned disadvantages and by means of which porous organic layers can be applied to a substrate in a simple and reproducible manner.
According to the invention, this object is achieved by a process for producing a porous layer adhering to a substrate, which comprises the steps:                a. preparation of a composition comprising an organic polymer constituent and also an inorganic-organic constituent and/or an inorganic constituent,        b. application of this composition to a substrate and formation of a layer on the substrate, and        c. removal of the inorganic-organic constituent and/or the inorganic constituent from the layer to form a porous layer adhering to the substrate.        
Further and preferred embodiments are disclosed in the description below and in the claims.